1. Field of the Invention
This invention relates to fuel-air mixers for gas turbine engine combustors and, more particularly, to reducing the formation of solid carbon or coke on such fuel-air mixers.
2. Description of Related Art
Gas turbine engine combustors use fuel nozzles and fuel-air mixers for mixing and burning fuel with compressed air. The fuel is typically premixed with air in the fuel-air mixers prior to combustion in order to minimize smoke and other undesirable by-products and to maximize the efficiency of the combustion process.
Fuel-air mixers are designed to atomize the fuel and to premix it with air in order to produce efficient and complete combustion. Low pressure fuel-air mixers have been designed which incorporate primary and secondary counter-rotational air swirlers which atomize fuel by the high shear forces developed in the area or zone of interaction between counter-rotating air flows produced by the primary and secondary air swirlers. An air swirler, also referred to as a swirler cup, includes a venturi and circumferentially and downstream angled air jets formed around an axis of the venturi. The air jets swirl the air prior to intermixing with the fuel to enhance atomization as well as mixing.
A very common problem with fuel-air mixers is the formation of carbon, commonly referred to as coking on combustor parts and, in particular, venturis of the air swirlers. Solid carbon or coke is formed by impingement of liquid hydrocarbon fuel on hot metal surfaces. This results in thermal decomposition of the fuel and precipitation of solid carbon or coke on the surface. Coke is typically formed at temperatures between 400 and 900 degrees F., which is typical of the combustor inlet conditions of a modern gas turboshaft or turbofan engine. Solid carbon will oxidize or burn away at temperatures in excess of 900 degrees F.
Although these temperatures are seen during high power operation, the cooling effect of the liquid fuel impingement prevents the venturi surface from reaching temperatures high enough to allow the carbon to burn away. Tests on instrumented venturis have shown surface temperatures to be 300 to 400 degrees F. below the inlet air temperature, which results in the venturi surface being in the 400–900 degrees F. carbon formation window for most of the engine operation. The impingement of liquid fuel also prevents oxygen from reaching the surface, further contributing to carbon buildup.
The formation of carbon on the venturi surface distorts the aerodynamic shape of the surface thereby disrupting the distribution of fuel in the combustor. This results in combustor hot streaks and resulting turbine distress. The combustor temperature distortions also distort the exit temperature thermocouple readings used to monitor engine deterioration, resulting in false deterioration indications. Engine starting and altitude ignition have also been shown to be adversely affected. In severe cases, these carbon deposits have caused total blocking of the venturi passage causing fuel to be deposited outside the combustor liner, and causing casing burn-through and in flight shutdown.
Disclosed in U.S. Pat. No. 6,571,559 is a fuel nozzle positioned inside the upstream end of a radial inflow primary swirler and adjacent to the venturi, a fuel passage through the fuel nozzle from which fuel is sprayed into the venturi at a designated spray angle and, a purge airflow circumscribing the fuel passage. The purge airflow flowing substantially parallel to a longitudinal axis of the venturi to provide a boundary layer of air along the inner surface of the venturi. The boundary layer of air minimizes the amount of fuel contacting the inner surface of the venturi subsequently reducing carbon formation. Annular passages or air shrouds have been incorporated into the fuel injector tip of the fuel nozzle to admit non-swirling air for the purpose of suppressing carbon formation (see U.S. Pat. Nos. 6,571,559 and 5,123,248 as examples). The air shrouds in the fuel nozzle tips cannot always be accommodated in the fuel nozzle tips.